Fix some ordered comparisons with zero for pointers.

Bug: http://b/31532493

The latest clang will trigger an error diagnostic "ordered comparison
between pointer and zero ..." when you do something like "p > 0". This
change replaces the ordered comparisons with proper checks instead.

Test: Validated with latest toolchain.
Change-Id: I4a127fe2551688c8176bdd31210cf7f2f40baeba